1. Field of the Invention
The present invention relates to a system for detecting changes in body posture.
2. Description of the Prior Art
Most sensors for measuring different physiological parameters of a subject, like pressure, electrical impedance etc., are affected by changes in the body posture of the subject. Thus more accurate and reliable information can be obtained if the body posture is known. In U.S. Pat. No. 5,370,667 a device and a method are described for automatically adjusting tachycardia recognition criteria based on detected physical activity of the patient. In this way it is possible to discriminate between physiological and pathological tachycardias. The activity sensor, which is an accelerometer of a piezoelectric, piezoresistive or piezocapacitive type, determines the activity status of the patient, including the position of the patient, and this information is used to adjust a threshold rate for the tachycardia recognition criterion of an ECG.
Furthermore, the body responds to a change in body posture from supine to standing by a transient increase in the heart rate (see U.S. Pat. No. 5,354,317). In this patent an apparatus and a method for cardiac pacing responsive to patient position are described, the same types of accelerometer as mentioned above being proposed for detecting changes in posture of the patient. This information is used for controlling the pacing rate, in a manner which is as physiologically correct as possible.
It has now been observed that body posture changes result in immediate changes in the morphology of the ECG. Thus FIG. 1 shows average ECGs for a number of cardiac cycles with the patient in the supine position and in a sitting upright position, respectively. As can be seen from this FIG. 1, the two positions of the patient are reflected in a characteristic difference in the surface ECG after the QRS complex. FIGS. 2 and 3 respectively illustrate an average IECG of two patients for a number of cardiac cycles for three different positions, standing, sitting and supine. Also in FIGS. 2 and 3, characteristic changes in the average IECGs can be observed for the different body postures of the patient, these differences being more pronounced in certain portions of the cardiac cycle then in other portions.